There is known a spark plug in which a needlelike spark member is provided on an inner surface (one surface) of the distal end portion of a ground electrode which faces a center electrode, so as to form a spark discharge gap between the spark member and the center electrode. As compared with a conventional spark plug, in the spark plug having such a needlelike spark member, the ground electrode can be located more distant from the spark discharge gap. Accordingly, a flame nucleus to be formed in the spark discharge gap is unlikely to come into contact with the ground electrode in the initial stage of its growth. Thus, since a so-called flame-extinguishing action; i.e., hindrance of growth of a flame nucleus due to removal of heat caused by contact of the flame nucleus with the ground electrode, can be weakened, the ignition performance of the spark plug can be improved.
There is known a spark plug (refer to, for example, Patent Document 1) in which such a spark member (chip-attached intermediate member) is composed of a noble metal member (chip) and an intermediate member, which is joined to a ground electrode. According to Patent Document 1, a bottom surface (second surface) of the intermediate member which is joined to the ground electrode is rendered wider than a top surface (first surface) of the intermediate member which is joined to the noble metal, whereby the area of welding is expanded for enhancement of joining strength. By virtue of this, the spark member and the ground electrode can be joined by means of generally practiced resistance welding.
Meanwhile, laser welding is generally employed for joining the noble metal member and the intermediate member. A weld metal zone formed in a joint of the noble metal member and the intermediate member is generally lower in strength than the noble metal member and the intermediate member. Thus, in the course of resistance welding of the spark member to the ground electrode, application of a pressing force to the intermediate member via the noble metal member for bringing the junction surfaces of the intermediate member and the ground electrode in close contact with each other involves the risk of deformation of the weld metal zone stemming from increase in internal stress of the weld metal zone. Also, upon subjection to thermal load associated with use of the spark plug, the residual internal stress may lead to cracking, separation, or the like. In order to prevent such a problem, as disclosed in Patent Document 1, a pressing force is applied to a flange portion which is provided for implementing a bottom surface wider than the top surface, so as to avoid applying the pressing force to the noble metal member. In this state, while the bottom surface of the intermediate member and the inner surface of the ground electrode are brought in close contact with each other, resistance welding is carried out.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2004-134209